This invention relates to integrated circuit substrates and masks.
Integrated circuits are made by selectively diffusing or implanting dopants into areas on the surface of a semi-conductor material, such as silicon, so as to modify the conductivity of the doped areas. Layers of insulating material, such as silicon oxide, and conducting material, such as aluminium, are then successively deposited on the surface to insulate and interconnect different areas. At each step in the manufacturing process, the areas into which the dopants and surface layers are introduced are defined by means of a resist coating. Selected areas of the resist coating are exposed to electromagnetic radiation and then developed, or washed, to leave either the areas of exposed or unexposed resist, depending on the type of resist. These remaining areas of resist mask the underlying areas of the substrate during the doping or other process, so that only those areas of the substrate exposed through windows in the resist layer will be doped or otherwise treated.
The resist layer can be exposed to radiation in one of two different ways. One method uses an electron beam which is focussed to a small spot on the wafer and raster scanned across it, being modulated on or off as it does this so as to expose the appropriate areas. The other, more commonly used method employs a mask bearing the desired pattern which is placed over the resist layer. Electromagnetic radiation such as ultra-violet radiation is used to illuminate the mask so that only those regions of the resist under transparent areas of the mask are exposed to radiation. The mask is subsequently removed and can be reused.
The typical integrated circuit is much smaller than the semiconductor substrate which is in the form of a circular wafer of diameter between about 50 mm and 100 mm. The circuit formed on the substrate is in the form of a rectangular die, each substrate generally holding several hundred dice arranged in parallel rows across the substrate, although, depending on the size of the dice and wafer this number may be between about thirty and several thousand.
The material costs involved in manufacture of such circuits is relatively low, the major manufacturing costs arising from the preparation of the mask. This is prepared by optically transferring a pattern of integration drawn to a large scale on paper, at a reduced scale, to a mask in the form of a transparent plate having a photographic emulsion layer. In order to keep manufacturing costs down, this optical transfer process is repeated by stepping the mask across its width in parallel rows to expose adjacent areas of the mask to an identical pattern of integration. In this way, all the dice in any one substrate will be identical. It will be appreciated that if different dice are to be mixed on one mask, this will considerably add to the cost of producing the mask, since it will be necessary to change repeatedly the pattern to which the mask is exposed during manufacture. The cost and difficulties of doing this are so great that it is generally preferable to prepare separate masks for each different die even though this may lead to considerable waste when only small numbers of one die are required. As a compromise, masks have been produced in which one or more rows of dice have a different configuration of dice from the dice in the other rows, although each die in any row is identical. Even this, however, requires added complication to the mask manufacturing process leading to additional costs. Furthermore, where only small numbers of any one type of die is required there will still either be considerable waste, if each substrate only has a small number of different dice, or excessive costs if there are a larger number of rows each with a different die.
Because each substrate is subjected to a number of different processing steps during manufacture, each of which require the substrate to be exposed to different patterns for doping, insulation, interconnection and so on, each of these processes requires a different mask dedicated to that process. This contributes to the cost of manufacture of the integrated circuits, being especially significant where only small numbers of circuits are required.
It is an object of the present invention to enable a reduction in these manufacturing costs especially in small quantity production.
According to one aspect of the present invention there is provided a method of manufacture of an integrated circuit substrate comprising a plurality of different independent circuit dice, each die being repeated a plurality of times over the substrate, a representation of the pattern of integration of the different dice being produced as a composite representation of a plurality of dice, the composite representation being transferred to a mask repeatedly to reproduce the composite representation and each die in a different region of the mask on each transfer, and the plurality of composite representations on the mask being transferred to the substrate simultaneously such as to reproduce the pattern of integration of the dice at a plurality of locations in the substrate.
In this way, a mask can be produced having several different dice without increasing its cost, since it is only necessary to transfer the same composite representation to the mask.
According to another aspect of the present invention there is provided a method of manufacture of a mask for use in the manufacture of an integrated circuit substrate of the kind comprising a plurality of different independent circuit dice, each die being repeated a plurality of times over the substrate, a representation of the pattern of integration of different dice being produced as a composite representation of a plurality of dice, and the composite representation being transferred to the mask repeatedly to reproduce the composite representation and each die in a different region of the mask on each transfer, so that the plurality of composite representations can be transferred simultaneously to the substrate by imaging the mask on the substrate.
The composite representations on the mask may be transferred to the substrate by passing radiation through the mask to expose a layer of resist on the substrate.
The area of the mask covered by the composite representations may be greater than the usable area of the substrate such that for at least two of the composite representation at least one of their respective dice is not fully reproduced on the substrate, at least one of those of the dice not fully reproduced on the substrate in the other of the two composite representations. The method may include the steps of selecting those of the dice fully reproduced on the substrate and rejecting those of the dice not fully reproduced on the substrate.
The mask may have two regions over its area associated with respective different processing steps, the pattern of integration in both the regions being transferred simultaneously to respective areas of the substrate, the substrate being subsequently subjected to one of the different processing steps associated with one of the regions, and the mask or an identical mask being used to transfer the pattern of integration in the other of the regions to a substrate which is subsequently subjected to the other of the processing steps. The mask or an identical mask may be used to transfer the pattern of integration in the other of the said regions to the same substrate. The mask or an identical mask may be rotated relative to the substrate after the one processing step and prior to the other of the processing steps.
One region of the mask may have a pattern of integration associated with a processing step for one substrate, another region of the mask having a pattern of integration associated with a processing step for a different substrate, and the mask or an identical mask being used to transfer both said patterns of integration to the two different substrates.
Reproductions of identical composite representations on the mask are preferably aligned with one another, and corresponding dice in aligned identical composite representations are optically compared via respective optical systems.
According to a further aspect of the present invention there is provided a substrate made by the above one aspect of the present invention. According to yet another aspect of the present invention there is provided a mask made by the above other aspect of the present invention.
Some at least of the composite representations may be rectangular or square in shape. Some of the composite representations may differ from others of the composite representation.
The mask may include a first group of composite representations comprising two rows of three composite representations each, a second group of three composite representations different from the composite representations of the first group arranged as a third row adjacent one of the said two rows, the size of the mask relative to the substrate being such that all the composite representations in the central row of the three rows, in the first group, will be fully reproduced on the substrate, that one composite representation in the other row of the first group and in the second group will be fully reproduced, and that two composite representations in the other row of the first group and in the second group are partially reproduced, with those dice reproduced from the two partially reproduced composite representations together forming one complete composite representation in each group.
The mask may have three identical copies of each die in each composite representation of the second group. Some at least of the composite representations may be of a different size from others of the composite representations. Some of the dice in the composite representations may be identical with other dice in the same composite representation.
Alternatively, all the dice in the composite representations may be different from other dice in the same composite representation.
Some at least of the dice in the composite representations may differ in size from others of the dice in the same composite representations.
The dice preferably have alignment marks thereon. The dice may have contact pads arranged around their edges, the contact pads being arranged at identical locations on different dice. The dice may be electrically encoded.
Methods of manufacturing integrated circuit substrates and masks used in their manufacture in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings.